I. Field of the Invention
The present invention relates to an extrusion head for the manufacturing of a hollow string of material and, in particular, to an extrusion head which manufactures a two-layered string whose joining seams are circumferentially staggered relative to one another.
II. Description of the Prior Art
An extrusion head (West German Pat. No. DT-PS 1,218,708) is known which, in order to manufacture a hollow string of material consisting of two concentric layers, is equipped with two inlet passages located diametrically opposite to one another and perpendicular to the axis of the extrusion head, each inlet passage being fed by its own extruder. The flows of material supplied to the inlet passages are formed into a tube by deflecting them through 90.degree., the tube forming a seam at the point, opposite the inlet passage, where the flows meet. The two tubes are separated by a sleeve in the extrusion head and united at exit from the head to form a common, two-layered hollow string of material. Because the two inlet passages are disposed diametrically opposite one another, the joining seams are circumferentially staggered through 180.degree. relative to one another. Again, it is known (West German Pat. No. DT-PS 1,082,043 ) to connect the two diametrically opposite inlet passages by means of a semi-circular distribution passage supplied centrally by an extruder.
West German Pat. No. DT-PS 1,704,799 discloses an extrusion head having a transversely disposed inlet passage on its inside. This patent also discloses a device for deflecting and distributing the thermoplastic material to a round chamber extending axially vis a vis the extrusion head and having an annular shape due to the presence within it of an axially displaceable annular piston. A sleeve is disposed concentrically vis a vis an axially displaceable die core belonging to the extrusion head, which sleeve forms with its conical end (which is disposed towards the die slot of the extrusion head) an opening for the passage of the thermoplastic material into the ram chamber. This patent also discloses a device for axially displacing the annular piston and a device for axially displacing the die core. To ensure that constant, uniform flow of the plasticized material into the ram head takes place, irrespective of the viscosity and the delivery rate, the device for deflecting and distributing the thermoplastic material consists of an annular groove surrounding the sleeve into which the inlet passage opens and whose internal wall is formed by the sleeve itself, the latter being axially displaceable upon the annular piston relative to same, thereby acting as a stripper. The transfer slot is designed as a variable restrictive slot located between the annular groove and the ram chamber and having a smaller cross section than the annular groove. The advantage of this known extrusion head resides in the fact that, because of the facility for variation of the restrictive slot, the overall cross section of the latter can be adjusted to a particular delivery rate and a specific viscosity, so that adjustment of the annular slot ensures a uniform peripheral mass flow into the ram chamber irrespective of the delivery rate and viscosity of the plasticized material.